9.3 — Common semantic errors in C++

In this lesson, you'll learn to identify and avoid common semantic errors - bugs that occur when your code compiles successfully but doesn't behave as intended.

What are semantic errors?

Unlike syntax errors (which prevent compilation), semantic errors are mistakes in program logic. Your code compiles and runs, but produces incorrect results or unexpected behavior.

Think of semantic errors like following a recipe with the wrong measurements. The recipe (code) is readable and you can follow it (it compiles), but the cake doesn't taste right (wrong output).

Common types of semantic errors

1. Off-by-one errors

One of the most frequent semantic errors involves incorrect loop bounds or array indexing:

#include <iostream>
#include <vector>

int main()
{
    std::vector<int> numbers = {10, 20, 30, 40, 50};

    // Wrong: accessing beyond array bounds
    for (int i = 0; i <= numbers.size(); ++i)
    {
        std::cout << numbers[i] << " ";
    }

    return 0;
}

Problem: Using <= instead of < causes accessing numbers[5], which doesn't exist.

Correct version:

#include <iostream>
#include <vector>

int main()
{
    std::vector<int> numbers = {10, 20, 30, 40, 50};

    // Correct: proper bounds checking
    for (int i = 0; i < numbers.size(); ++i)
    {
        std::cout << numbers[i] << " ";
    }

    return 0;
}

Output:

10 20 30 40 50

2. Uninitialized variables

Using variables before giving them a value leads to unpredictable behavior:

#include <iostream>

int main()
{
    int total;  // Uninitialized variable
    int count = 5;

    // Wrong: using uninitialized variable
    total = total + count;  // total has garbage value

    std::cout << "Total: " << total << std::endl;

    return 0;
}

Possible output (unpredictable):

Total: 32773

Correct version:

#include <iostream>

int main()
{
    int total = 0;  // Initialize variable
    int count = 5;

    total = total + count;

    std::cout << "Total: " << total << std::endl;

    return 0;
}

Output:

Total: 5

3. Integer division when you want floating-point

Integer division truncates the result, which may not be what you intended:

#include <iostream>

int main()
{
    int numerator = 5;
    int denominator = 2;

    // Wrong: integer division
    double result = numerator / denominator;
    std::cout << "5 / 2 = " << result << std::endl;

    return 0;
}

Output:

5 / 2 = 2

Correct version:

#include <iostream>

int main()
{
    int numerator = 5;
    int denominator = 2;

    // Correct: cast to double for floating-point division
    double result = static_cast<double>(numerator) / denominator;
    std::cout << "5 / 2 = " << result << std::endl;

    return 0;
}

Output:

5 / 2 = 2.5

4. Assignment vs. comparison

Using = (assignment) instead of == (comparison) in conditionals:

#include <iostream>

int main()
{
    int score = 85;

    // Wrong: assignment instead of comparison
    if (score = 100)  // This assigns 100 to score!
    {
        std::cout << "Perfect score!" << std::endl;
    }

    std::cout << "Score is now: " << score << std::endl;

    return 0;
}

Output:

Perfect score!
Score is now: 100

Correct version:

#include <iostream>

int main()
{
    int score = 85;

    // Correct: comparison operator
    if (score == 100)
    {
        std::cout << "Perfect score!" << std::endl;
    }
    else
    {
        std::cout << "Score: " << score << std::endl;
    }

    return 0;
}

Output:

Score: 85

5. Infinite loops

Loops that never terminate due to incorrect loop conditions:

#include <iostream>

int main()
{
    int i = 0;

    // Wrong: condition never becomes false
    while (i >= 0)
    {
        std::cout << i << " ";
        ++i;  // i keeps growing, always >= 0

        // Safety break for demonstration
        if (i > 10) break;
    }

    return 0;
}

Output:

0 1 2 3 4 5 6 7 8 9 10

Correct version:

#include <iostream>

int main()
{
    int i = 0;

    // Correct: loop terminates when i reaches 5
    while (i < 5)
    {
        std::cout << i << " ";
        ++i;
    }

    return 0;
}

Output:

0 1 2 3 4

6. Dangling else problem

Incorrect if-else structure leading to unexpected behavior:

#include <iostream>

int main()
{
    int temperature = 75;
    bool isSunny = false;

    // Misleading indentation - else belongs to inner if!
    if (temperature > 70)
        if (isSunny)
            std::cout << "Perfect weather!" << std::endl;
    else
        std::cout << "Too cold!" << std::endl;

    return 0;
}

Output: (No output - the else is associated with the inner if)

Correct version:

#include <iostream>

int main()
{
    int temperature = 75;
    bool isSunny = false;

    // Use braces to make structure clear
    if (temperature > 70)
    {
        if (isSunny)
        {
            std::cout << "Perfect weather!" << std::endl;
        }
    }
    else
    {
        std::cout << "Too cold!" << std::endl;
    }

    // Or restructure the logic
    if (temperature > 70 && isSunny)
    {
        std::cout << "Perfect weather!" << std::endl;
    }
    else if (temperature <= 70)
    {
        std::cout << "Too cold!" << std::endl;
    }
    else
    {
        std::cout << "Warm but not sunny" << std::endl;
    }

    return 0;
}

Output:

Warm but not sunny

7. Incorrect operator precedence assumptions

Assuming wrong order of operations:

#include <iostream>

int main()
{
    int x = 5;
    int y = 3;

    // Wrong assumption: thinking && has higher precedence than ==
    if (x > 0 && y == 3 == true)  // Actually: (y == 3) == true
    {
        std::cout << "Condition met" << std::endl;
    }
    else
    {
        std::cout << "Condition not met" << std::endl;
    }

    return 0;
}

Output:

Condition met

Correct version:

#include <iostream>

int main()
{
    int x = 5;
    int y = 3;

    // Clear and explicit
    if (x > 0 && y == 3)
    {
        std::cout << "Both conditions met" << std::endl;
    }

    // Or if you really need to check for true
    if (x > 0 && (y == 3) == true)
    {
        std::cout << "Explicit comparison to true" << std::endl;
    }

    return 0;
}

Output:

Both conditions met
Explicit comparison to true

Strategies for preventing semantic errors

1. Initialize all variables

// Good practice
int count = 0;
double total = 0.0;
bool isValid = false;

2. Use meaningful variable names

// Better
int studentCount = 0;
double averageGrade = 0.0;

// Rather than
int c = 0;
double a = 0.0;

3. Always use braces with control structures

// Good - clear structure
if (condition)
{
    doSomething();
}
else
{
    doSomethingElse();
}

4. Be explicit with operator precedence

// Clear intent
if ((a > b) && (c < d))
{
    // action
}

5. Test boundary conditions

Always test edge cases like:

• First and last elements of arrays
• Empty containers
• Zero values
• Maximum and minimum values

Debugging semantic errors

1. Use a debugger: Step through code line by line
2. Add print statements: Output variable values at key points
3. Test with simple inputs: Use known values to verify logic
4. Rubber duck debugging: Explain your code to someone (or something)
5. Code review: Have someone else look at your code

Summary

Semantic errors are logic mistakes that produce incorrect behavior even though the code compiles successfully. Common types include off-by-one errors, uninitialized variables, integer division issues, assignment vs. comparison confusion, infinite loops, and operator precedence problems. Prevention strategies include proper initialization, meaningful names, explicit structure, and thorough testing.

Quiz

1. What's the difference between a syntax error and a semantic error?
2. Why are semantic errors often harder to detect than syntax errors?
3. What happens when you use an uninitialized variable?
4. How can you avoid the "dangling else" problem?
5. What's wrong with this code: if (x = 5) in a conditional statement?

Practice exercises

1. Find and fix the semantic error in this code:

int sum = 0;
for (int i = 1; i <= 10; i++)
{
    sum = i;  // Bug here
}

2. Identify the problem with this average calculation:

int total = 100;
int count = 3;
double average = total / count;

3. Fix this loop that's supposed to count down from 10 to 1:

for (int i = 10; i > 0; i++)
{
    std::cout << i << " ";
}

4. Debug this function that should return the maximum of two numbers:

int getMax(int a, int b)
{
    if (a > b)
        return a;
    else
        return a;  // Bug here
}